Multiple coil assembly



Jam 1935- w. D. LOUGHLIN ET AL 1,989,205

MULTIPLE COIL ASSEMBLY Filed Sept. 16, 1931 2 Sheets-Sheet 'l Jan. 29, 1935. w LQUGHLIN ET AL 1,989,205

MULTIPLE COIL ASSEMBLY Filed Sept. 16, 1931 2 Sheets-Sheet 2 Patented Jan. 29, 1935 UNITED STATES PATENT OFFICE MULTIPLE COIL ASSEMBLY Application September 16, 1931, Serial No. 563,180

This invention relates to multirange tuning induotances and particularly to inductances adapted to form a part of the coupling circuits in radio receivers that are to be tuned over a plurality of ranges of signal frequencies.

The usual types of tapped inductanccs, whether simple inductances or transformers, have introduced substantial losses in radio frequency circuits when the switch was adjusted to include only a portion of the tapped inductance in the circuit. To avoid these losses, it has been customary to employ sets of interchangeable coils ortransformers to cover the individual frequency bands.

An object of the invention is to provide a tapped inductance which does not exhibit substantial losses when a part only of the inductance is effectively included in a resonant circuit. Another object is to provide a coupling system including a tapped inductance or a transformer having a tapped winding, and which does not exhibit substantial losses when the system is adjusted for transmission of the higher frequencies. More specifically, objects are toprovide a tapped inductance, or a transformer having a tapped winding, in which the tapped inductance or winding has the form oia single layer solenoid of constant diameter, but in which the sections, for resonating at the higher frequency band or bands are formed of larger diameter wire and have a larger pitch than the sections which are employed when tuning to lower frequency bands.

These and other objects of the invention will be 1 apparent from the following specification when taken with the accompanying drawings, in

1 In the circuit diagrams, Figs. 1 and 2, the referenee'numerais l and 2identify, respectively, twoamplifier tubes in a cascaded and tuned radio frequency amplifier. Except for the provision of the tapped inductances, Fig. 1 will be recongm'zed as'atypical tuned impedance coupling type of amplifier in which the plate circuit of the tube 1 includes, in parallel, an inductance L and tuning condenser C, and the amplified signal is impressedupon the succeeding tube 2 through the coupling condenser 3. Similarly, the coupling system of the Fig. 2 circuit is of the tuned transformer type, including a primary winding L1 in the plate circuit of tube 1 and a secondary winding L2, shunted by the tuning condenser C2, in the control grid circuit of the succeeding tube 2.

It will be understood that these types of coupling systems are illustrative of applications of the invention and, in general, form no part of the same.

As indicated diagrammatically in Fig. 1, the inductanceL may be, and preferably is, a solenoidal winding of uniform diameter but of non-uniform pitch. Leads 4 extend from the taps which subdivide the continuous winding of inductance L into sections L; to Le, to switch contacts that are engageable by a switch arm 5, and the higher frequency section La of the inductance is of substantially greater pitch than the section Le which is at the opposite end of the inductance. The switch arm 5 is grounded, for signal frequency, by a by-pass condenser 6, and it is to be noted that the switch arm 5 short-circuits the low frequency sections which are not included in the resonant circuit. With the switch arm in the position shown in Fig. 1, only the sections La and L are included in the resonant circuit, the remaining sections being individually short-circuited and grounded for signal frequencies. For tuning over the highest frequency band, only the section La is employed, and additional sections are included in series with La when receiving signals in the lower frequency bands.

It is also to be noted, as indicated diagrammatically in Fig. 1, that the physical arrangement of inductance L and the switch mechanism is such that the shortest leads extend between the switch and the higher frequency sections of the inductance.

For transformer coupling, as illustrated in Fig. 2, the secondary winding L2 is tapped and, for the same frequency bands, may be substantially identical, in physical form, with the simple inductance L that would be appropriate for a tuned impedance coupling. The primary winding L1 is formed of insulated wire of uniform and small diameter throughout its entire length but varice in pitch with the secondary winding. The transformer thus comprises a twin winding, i. e. the primary and secondary are wound turn for turn, and in close mechanical contact, throughout the length of the respective windings. Substantial unity coupling between the windings is therefore obtained, and there is substantially no diilerence in potential between adjacent portions of the primary and secondary windings; conserality of tuned stages are employed, it will beapparent that other types of switches may. be employed. V

Due to the unity coupling, it will be apparent that the short-circuiting of one or more sections. of the secondary winding L2 effectively short-cit cuits the corresponding section or sections of the primary winding L1. p I While this short-circuiting of that portion fromthe plate circuit of tube 1, it will be apparent that the resistance of the shortcircuited section is not eliminated from winding L1. The low potential end of the winding L1 is grounded, for signal frequencies, by condenser 8, and the relatively heavy flow of oscillatory currents in the tuned secondary circuit establishes a current flow in the primary circuit which includes the winding L1, by-pass condenser 8 and the inherent plate-cathode capacity Cp of tube 1 The resistance of the short-circuited. portion of the primary winding L1 is therefore reflected into the secondary circuit and is particularly troublesome at the higher frequency bands.

To prevent this undesired introduction of resistance into the secondary circuit, thelowimpedance condenser 9 is connected directly across the high potential ends of the transformer windings. No loss or other disturbing effects results from this high capacity coupling since the unity coupling between the windings results in identical potentials, and therefore no tendency for current flow, across the high potential ends of the windings. Due, however, to the relatively high re:- sistance of the winding L1, the major portion of the current flowing in the inherent capaoityC does not pass through thewinding L1 but through the low resistance winding L2. Since practically no current flows through the high resistance pri-,

mary, there is substantially no resistance reflected into the secondary winding.

A physical embodiment of a transformer for use with a 200 micromicrofarad variable condenser for tuning over the frequency band of from 500 to 15,000 kilocycles is illustratedin Fig. 3.

The form 10 for supporting the windings is of.

molded insulating material which is preferably grooved, as indicated at 11, Fig. 4, to facilitate the positioning and the stability of the windings. The form 10 has an integral base or flange 12 for facilitating the mounting of the transformer on a panel or chassis, and terminals 13 are secured in the flange.

The fine wire 14, N0. 36 double silk covered, which constitutes the transformer primary vis wound upon the form 10, and the secondary wind ing is then wound in place, with taps 15 taken off at appropriate intervals and led through the interior of the form to the respective terminals 13 on flange 12. In the particular transformer illustrated in Fig. 4, the outside diameter of the form 10 was 1% inches, and the total length of the winding was about 4% inches. The section La is adjacent the flange 12 and comprises 7 turns of No. 16 bare copper wire, wound, 12 turnsper' a portion of the primary winding L1 removes the inductance of inch, a gap of A; inch being provided between this and the Lb section which comprises 12 turns of No. 18 bare copper wire wound 16 turns per inch. Section Lc is also of bare copper wire, No. 24, and consists of 25 turns wound with a pitch of 32 turns per inch. Section La has 56 turns, close wound, of No. 26 double silk covered wire and section Le has 112 turns," close wound, of No. 32 enameled wire. The transformer "may be shielded by enclosing the same in a 2 inch copper can.

By omitting the primary winding, wire 14, a

tapped inductance of the design specified will circuits or arrangements where it is desired to tune a circuit to. resonance over the band of from 500' to 15,000 kilocycles.

. While the tapped inductance does not have, for any frequency band, the efiiciency which could be obtained'with a coil of the same diameter and designed for only a singlerange, the losses introduced are not prohibitive and are outweighed by advantages derived from the use of a simple switching system instead-of replaceable coils.

For example, the high frequency section La of a tapped inductance such as formed by only the secondary winding, Fig. 3, and having sectionswound as specified above, will have an average figure of merit over the 7000to 15,000 kilocycle band that is about 83% of the figure of merit of an inductance designed to cover'that, particular band. By figure of merit is meant the ratio of the coil reactanoe to resistance, 1

At 7000 kilocycles, the comparative valueof the v tapped coil was 81%, at 10,000 kilocyc1es it was 88.5%, and at 15,000 kilocycles it was 85.5%.

. The exact construction which will give best resultswill vary with the particular band of frequencies which is to be coveredpthe number ofsections into which the inductance is to be divided, the design of the tuning condenser, and with other factors which will berapparent to those familiar with the design of highfrequency circuits.

We claim:

a1. In ;a selective vacuum tube amplifier, a tuned circuit comprising a condenser and an inductance connected in parallel, means ground ing one joined terminal of said condenser and inductance at the frequencies of operation, said inductance being divided into sections by taps, and a switch having points located adjacentthe non-grounded terminal of said inductance, said points being connected to ,therespective'taps, and a grounded contact arm for said switch, said contact arm being movable toprogressively short circuit and ground successive. sections of said.

inductance. i

2. The invention as set forth in claim 1, where-v in said inductance is the secondary winding of a transformer,- and the primary winding thereof is wound turn forgturnwith and closelyadjacent the corresponding turns of the said inductance. H 1

3, In an amplifier stage, a tunable circuit comprising-a variable condenserand an inductance coil having a substantially solenoidal continuous; winding, an end portion of said winding consisting of larger diameter wire and having larger pitch 'than the opposite end portionpf said Windina.

4. A high frequency transformer for use in an amplifier stage adapted for operation in each of a plurality of frequency bands, said transformer comprising a cylindrical form, primary and secondary windings wound turn for turn on said form as single layer solenoids, one adjacent pair of terminals of said windings constituting the high potential terminals of the respective windings, and taps dividing one of said windings into sections, the pitch of the windings being constant throughout each section and the pitch of the section adjacent the high potential terminals being greater than that of the opposite end of the windings, means for short-circuiting sections of said tapped windings progressively from the low potential terminal thereof, and a tuning condenser shunted across said tapped winding.

5. The invention as claimed in claim 4, in combination with a condenser connected between the said high potential terminals of the respective windings.

6. The invention as claimed in claim 4, wherein the primary winding is of relatively small wire of relatively high resistance, in combination with a condenser connected between the said high potential terminals of the respective windings.

'7. As an element of a radio-selective amplifier, a tuned circuit having two terminals, a condenser connected between said terminals, and a continuous solenoidal inductance coil connected between said terminals, said inductance coil having a plurality of sections of different conductor size, whereby at least one of said sections may be short-circuited without substantial electrical reaction on the remaining sections.

8. A radio-selective amplifier circuit includ ing as a selective coupling element the combination of a condenser and an inductance coil, said coil having a continuous solenoidal winding divided into sections, one section being wound with a conductor smaller than the conductors of the remaining sections, and a short-circuiting connection across the section wound with the smaller conductor.

9. In a radio-selective amplifier including as an element of a tuned circuit a solenoidal inductance having two end terminals and between said terminals a plurality of adjacent, series-connected sections, each section being wound with a conductor of size smaller than the conductor of the next adjacent section on one side, but larger than that of the next adjacent section on the opposite side, the method of reducing the total inductance between said end terminals which comprises shortcircuiting one of said end terminals to an intermediate point of said solenoidal inductance coil.

10. In an amplifier stage, the combination with a vacuum tube having an output electrode and a cathode, of a tuned circuit comprising a condenser having a low-potential terminal and an inductance connected in parallel with said condenser, said inductance being divided by taps into a plurality of sections, a switch for progressively shortcircuiting successive sections of said inductance and connecting both terminals of each shortcircuited section to the low-potential terminal of said condenser, an output inductance closely coupled to said tapped inductance and connected between said tapped inductance and cathode, whereby said output inductance is effectively in shunt with the inherent capacity between said electrode and cathode, and a capacitive coupling of low impedance between the electrode terminal of said output inductance and the adjacent terminal of said tapped inductance, said capacitive coupling maintaining the associated terminals of the two inductances at substantially the same alternating current potential.

11. An amplifier stage as claimed in claim 10, wherein said output inductance is coupled magnetically to said tapped inductance with substantially unity coupling.

12. The invention as set forth in claim 10, wherein said tapped inductance comprises a winding of substantially constant diameter, and the turns forming the section adjacent one end thereof are of greater pitch than the turns forming a section at the other end,

The invention as set forth in claim 10 wherein said tapped inductance comprises a winding of substantially constant diameter, and the turns forming the section adjacent one end thereof are of greater pitch and of larger diameter than the turns forming a section at the other end.

14. In an amplifier stage, the combination with a vacuum tube having an output circuit including an output-circuit coil connected between an output electrode and a cathode of said tube, said coil being effectively shunted by the inherent capacity between said output electrode and said cathode, of a tunable circuit comprising a variable condenser and an inductance coil having one open section, a second section which is adapted to be short-circuited, means connecting that terminal of said second section which is remote from said open section to the tube cathode to maintain the said terminal and the cathode at substantially the same potential for currents of the frequencies to be amplified, switch means for short-circuiting said second section, said second section being wound with a conductor of smaller diameter than the conductor of said open section, close electromagnetic coupling between said output-circuit coil and said inductance coil, and a capacitive coupling greater than the self-capacities of the coils between said output circuit coil and said inductance coil.

WILLIAM D. LOUGHLIN. PAUL O. FARNHAM. 

